1. Field of the Invention
This invention relates to production of hydrogen fluoride. More particularly, the invention relates to a process for producing hydrogen fluoride in a process for manufacturing volatile phosphorous compounds and their subsequent conversion to phosphoric acid.
2. Prior Art
In our patent applications there is taught a novel method for treating calcium phosphate sources such as bone phosphate and phosphate rock with fluorosulfonic acid, in the presence of a limited amount of water, to convert over 98 percent of the phosphorous to volatile compounds that can be subsequently hydrolyzed. This method is taught particularly in copending application Ser. No. 000,987, filed Jan. 4, 1979, now U.S. Pat. No. 4,202,867.
The method includes the hydrolysis of the volatile compounds to phosphoric acid and hydrogen fluoride. Hydrogen fluoride is then recycled to react with the required sulfur trioxide to make the requirement of fluorosulfonic acid for the process.
It was taught therein that dependent on the form of silica impurities in the ore, a net production of hydrogen fluoride could be obtained from the fluoride content of the rock. The process would be relatively self-sustaining as far as hydrogen fluoride used for making fluorosulfonic acid requirements would be concerned. Sulfur trioxide is essentially the net reagent required to treat the ore to produce the volatile phosphorous intermediates for subsequent conversion to phosphoric acid and hydrogen fluoride.
It had been thought that silicon dioxide would not be appreciably attacked by the fluorosulfonic acid so that a portion of the fluoride content of the ore could be released as hydrogen fluoride. It was recognized that the recycle of hydrogen fluoride to the process and the utilization of hydrogen fluoride produced from the fluoroapatite ore, to make up process losses, would be very pertinent to the economics of phosphoric acid manufacture.
Currently, essentially all hydrogen fluoride is manufactured by the treatment of fluorospar ore (calcium fluoride, CaF.sub.2) with sulfuric acid at about 250.degree. to 300.degree. C. It is felt that operating conditions must be carefully controlled to minimize the formation of an undesirable by-product, fluorosulfonic acid. Using the current methods, acid grade fluorospar must be low in silica, and is usually specified to contain no less than 97% CaF.sub.2. When silica, which is usually present in fluorospar and fluorapatite, reacts sulfuric acid is consumed and hydrogen fluoride production is lost. The following reaction illustrates this result: EQU 2CaF.sub.2 +SiO.sub.2 +H.sub.2 SO.sub.4 .fwdarw.2CaSO.sub.4 +SiF.sub.4 +2H.sub.2 O (1)
Theoretically, for each pound of silica present in the ore, 2.6 pounds of fluorospar is wasted and 3.3 pounds of sulfuric acid is consumed. SiF.sub.4 reacts with water, usually present, to form fluorosilicic acid, an objectionable by-product.